1. Field of the Invention
The invention relates to an electronic circuit, a sensor arrangement and a method for processing a sensor signal.
2. Description of the Related Prior Art
Such a sensor arrangement is known as a biosensor chip from R. Hintsche et al., Microbiosensors Using Electrodes Made in Si-Technology, Frontiers in Biosensorics, Fundamental Aspects, edited by F. W. Scheller et al., Dirk Hauser Verlag. Basel, pp. 267-283, 1997.
FIG. 2a and FIG. 2b show such a biosensor chip, as described in R. Hintsche et al., Microbiosensors Using Electrodes Made in Si-Technology, Frontiers in Biosensorics, Fundamental Aspects, edited by F. W. Scheller et al., Dirk Hauser Verlag. Basel, pp. 267-283, 1997. The sensor 200 has two electrodes 201, 202 made of gold, which are embedded in an insulating layer 203 of insulating material. Connected to the electrodes 201, 202 are electrode terminals 204, 205, to which the electronic potential applied to the electrodes 201, 202 can be supplied. The electrodes 201, 202 are arranged as planar electrodes. DNA probe molecules 206 are immobilized on each electrode 201, 202 (cf. FIG. 2a). The immobilization is carried out in accordance with the gold-sulfur coupling. The analyte to be investigated, for example an electrolyte 207, is applied to the electrodes 201, 202.
If DNA strands 208 with a sequence which is complementary to the sequence of the DNA probe molecules 206 are contained in the electrolyte 207, then these DNA strands 208 hybridize with the DNA probe molecules 206 (cf. FIG. 2b).
Hybridization between a DNA probe molecule 206 and a DNA strand 208 takes place only when the sequences of the respective DNA probe molecule 206 and of the corresponding DNA strand 208 are complementary to each other. If this is not the case, then no hybridization takes place. Thus, a DNA probe molecule with a predefined sequence is in each case only capable of bonding a specific DNA strand, specifically that one with a respectively complementary sequence, that is to say of hybridizing with it.
If hybridization takes place, then as can be seen from FIG. 2b, the value of the impedance between the electrodes 201 and 202 changes. This changed impedance is determined by applying an alternating voltage with an amplitude of approximately 50 mV to the electrode terminals 204, 205 and the current resulting from this by means of a connected measuring instrument (not illustrated).
In the event of hybridization, the capacitive component of the impedance between the electrodes 201, 202 is reduced. This can be attributed to the fact that both the DNA probe molecules 206 and the DNA strands 208, which possibly hybridize with the DNA probe molecules 206, are nonconductive and thus, as can be seen, shield the respective electrode 201, 202 electronically to a certain extent.
In order to improve the measurement accuracy, it is known from P. van Gerwen, Nanoscaled Interdigitated Electrode Arrays for Biochemical Sensors, IEEE, International Conference on Solid-State Sensors and Actuators, Chicago, pp. 907-910, 16.-19. Jun. 1997 to use a large number of electrode pairs 201, 202 and to connect these in parallel, these being arranged intermeshed with one another, as can be seen, so that the result is what is known as an interdigital electrode 300 (cf. FIG. 3). The dimension of the electrodes and the distances between the electrodes are of the order of magnitude of the length of the molecules to be detected, that is to say of the DNA strands 208, or below that, for example in the region of 200 nm and below.
Furthermore, principles relating to a reduction/oxidation recycling procedure for detecting macromolecular biopolymers are known from M. Paeschke et al., Voltammetric Multichannel Measurements Using Silicon Fabricated Microelectrode Arrays, Electroanalysis, Vol. 7, No. 1, pp. 1-8, 1996 and R. Hintsche et al., Microbiosensors using electrodes made in Si-technology, Frontiers in Biosensorics, Fundamental Aspects, edited by F. W. Scheller et al., Birkhauser Verlag, Basel, Switzerland, 1997. The reduction/oxidation recycling procedure, also designated the redox recycling procedure below, will be explained in more detail below using FIG. 4a to FIG. 4c. 
FIG. 4a shows a biosensor chip 400 having a first electrode 401 and a second electrode 402, which are applied to a substrate 103 as insulating layer.
A holding region, designed as a holding layer 404, is applied to the first gold electrode 401. The holding region serves to immobilize DNA probe molecules 405 on the first electrode 401.
No such holding region is provided on the second electrode.
If, by means of the biosensor 400, DNA strands with a sequence which is complementary to the sequence of the immobilized DNA probe molecules 405 are to be detected, then the sensor 400 is brought into contact with a solution 406 to be investigated, for example an electrolyte, in such a way that DNA strands possibly contained in the solution 406 to be investigated can hybridize with the complementary sequence to the sequence of the DNA probe molecules 405.
FIG. 4b shows the case in which the DNA strands 407 to be registered are contained in the solution 406 to be investigated and have hybridized with the DNA probe molecules 405.
The DNA strands 407 in the solution to be investigated are marked with an enzyme 408, with which it is possible to cleave molecules described below into part molecules.
It is usual to provide a considerably larger number of DNA probe molecules 405 than there are DNA strands 407 to be determined in the solution 406 to be investigated.
After the DNA strands possibly contained in the solution 406 to be investigated and having the enzyme 408 have hybridized with the immobilized DNA probe molecules 407, the biosensor chip 400 is rinsed, by which means the unhybridized DNA strands are removed and the biosensor chip 400 is cleaned of the solution 406 to be investigated.
This rinsing solution used for rinsing or a further solution supplied on its own in a further phase has an electronically uncharged substance added to it, which contains molecules which can be cleaved by the enzyme on the hybridized DNA strands 407, into a first part molecule 410 having a negative electronic charge and into a second part molecule having a positive electronic charge.
As FIG. 4c shows, the negatively charged first part molecules 410 are attracted to the positively charged anode, that is to say to the first electrode 401, as indicated by the arrow 411 in FIG. 4c. 
The negatively charged first part molecules 410 are oxidized at the first electrode 401, which has a positive electronic potential as anode, and are attracted as oxidized part molecules 413 to the negatively charged cathode, that is to say the second electrode 402, where they are reduced again. The reduced part molecules 414 again migrate to the first electrode 401, that is to say to the anode.
In this way, a circular electronic current is generated, which is proportional to the number of charge carriers respectively generated by the enzymes 408.
The electronic parameter which is evaluated in this method is the change in the electronic current   m  =            ⅆ      I              ⅆ      t      as a function of the time t, as illustrated schematically in the graph 500 in FIG. 5.
FIG. 5 shows the function of the electronic current 501 as a function of the time 502. The curve 503 which results exhibits an offset current Ioffset 504, which is independent of the time curve.
The offset current Ioffset 504 is generated by parasitic components on account of non-ideal states of the biosensor 400.
A substantial cause of the offset current Ioffset 504 resides in the fact that the covering of the first electrode 401 with DNA probe molecules 405 is not carried out in an ideal manner, that is to say not completely densely.
In the event of completely dense coverage of the first electrode 401 with DNA probe molecules 405, the result would be only purely capacitive electronic coupling between the first electrode 401 and the electronically conductive solution 406 to be investigated, because of what is known as the double-layer capacitance which is produced by the immobilized DNA probe molecules 405.
However, the incomplete coverage leads to parasitic current paths between the first electrode 401 and the solution 406 to be investigated, which inter alia also have non-reactive components.
However, in order to permit the oxidation/reduction process, the coverage of the first electrode 401 with the DNA probe molecules 405 may not be complete, in order that the electronically charged part molecules, that is to say the negatively charged first part molecules, are attracted to the first electrode 401 at all.
On the other hand, in order to achieve the greatest possible sensitivity of such a biosensor, in conjunction with low parasitic effects, the coverage of the first electrode 401 with DNA probe molecules 405 should be as dense as possible.
In order to achieve high reproducibility of the measured values determined by such a biosensor 400, both electrodes 401, 402 must always provide an adequately large area for the oxidation/reduction process in the context of the redox recycling procedure.
In the case of the biosensor according to the prior art, the result is thus a certain measurement uncertainty when determining the DNA strands in a solution to be investigated.
Macromolecular biopolymers are to be understood in the following text as, for example, proteins or peptides or else DNA strands with a respectively predefined sequence.
If proteins or peptides are to be registered as macromolecular biopolymers, then the first molecules and the second molecules are ligands, for example active substances with a possible binding activity, which bind the proteins or peptides to be registered to the respective electrode on which the appropriate ligands are arranged.
Possible ligands are enzyme agonists or enzyme antagonists, pharmaceuticals, sugars or antibodies or any kind of molecule which has the capability of bonding proteins or peptides specifically.
If the macromolecular biopolymers used are DNA strands with a predefined sequence, which are to be registered by means of the biosensor, then by means of the biosensor, DNA strands with a predefined sequence can be hybridized with DNA probe molecules with the sequence complementary to the sequence of the DNA strands as molecules on the first electrode.
Within the context of this description, a probe molecule is to be understood both as a ligand and as a DNA probe molecule.
The value m is proportional to the electrode area of the electrodes used to register the measurement current. Given a constant width of the electrodes, the value m is thus proportional to the longitudinal extent of the electrodes used, for example in the case of the first electrode 201 and the second electrode 202, to their length perpendicular to the plane of the drawing in FIG. 2a and FIG. 2b. 
If a plurality of electrodes are connected in parallel, for example in the known interdigital electrode arrangement, then the change in the measurement current m is also proportional to the number of respectively parallel-connected electrodes.
However, the value of the change in the measurement current m can have a very highly fluctuating value range, because of different influences, in particular for different solutions to be investigated.
The cause of the high fluctuations can firstly be the dynamic range required for DNA strands with a predefined sequence to be detected, in order to permit their registration at all.
Secondly, however, it is also possible that different macromolecular biopolymers of different types to be detected will lead to highly different value ranges for the measurement signal which results, that is to say in particular the measurement current and its change m over time, which in turn leads to a widening of the required overall dynamic range for a predefined electrode configuration with subsequent uniform measuring electronics, that is to say with subsequent uniform measuring circuit.
The measurement electronics, which register the time change between the electrodes, that is to say between anode and cathode, and process it further, must function reliably and accurately in the required value ranges.
The requirements on the large dynamic range of such a circuit lead to the measurement electronics being expensive and complicated, in order to provide the required dynamic range.
In other methods also, as are known for example from P. van Gerwen, Nanoscaled Interdigitated Electrode Arrays for Biochemical Sensors, IEEE, International Conference on Solid-State Sensors and Actuators, Chicago, pp. 907-910, 16.-19. Jun. 1997, WO 93/22678, DE 196 10 115 A1, C. Krause et al., Capacitive Detection of Surfactant Adsorption on Hydrophobized Gold Electrodes, Langmuir, Vol. 12, No. 25, pp. 6059-6064, 1996, V. Mirsky et al., Capacitive Monitoring of Protein Immobilization and Antigen-Antibody Reactions on Monomolecular Alkylthiol Films on Gold Electrodes, Biosensors & Bioelectronics, Vol. 12, No. 9-10, pp. 977-989, 1997, the case can occur in which the electronic measurement signals to be detected have to be measurable over a large dynamic range.
There, too, extreme requirements on the measurement electronics, that is to say the evaluation circuit, can result with respect to their dynamics. In particular during circuit design, the non-ideal states of the components used, that is to say noise, the variation in the parameters of the components, are taken into account in the form that, for these components, a working point in the circuit design is chosen at which these non-ideal states have the lowest possible influence on the quality of the measured result. If a circuit is to be operated over a large dynamic range, maintaining an optimum working point for all the ranges becomes increasingly more difficult and more complex.
Furthermore, in particular in the requirements illustrated above, the offset current Ioffset is very much greater than the time change of the measurement current m over the entire measurement period, that is to say measuring time is tmeas, which means that the following is trueIoffset>>m·tmeas.  (1)
It is therefore necessary, within a large signal (offset current Ioffset) to measure a very small time-dependent change (time change in the measurement current) with high accuracy.
The result is therefore very high requirements on the measuring instruments used.
The value of the parameter m=dI/dt can lie in a large value range for different analytes to be investigated, as presented above.
The cause for this can, firstly, be the dynamic range required for a species to be detected, but secondly it is also possible that different species to be detected lead from the start to very different value ranges for the resultant measurement signal, which in turn corresponds to a widening of the required overall dynamic range.
An electronic circuit which registers the time change of the circular current between anode and cathode and processes it further must therefore function reliably in the appropriate value range.
The requirements on the dynamic range of this circuit are therefore extremely high.
During circuit design, it is known to take into account the non-ideal states of the components used (noise, parameter variations) in the manner in which, for these components, a working point in the circuit is selected in which these non-ideal states play the smallest possible part.
If a circuit is to be operated over a large dynamic range, maintaining an optimum working point over all the ranges becomes increasingly more difficult, more complex and therefore more expensive, however.
Furthermore, the documents DE 196 09 621 C1, U.S. Pat. Nos. 5,726,597, U.S. Pat. No. 4,992,755, 4,987,379, 4,810,973, 4,495,470, 4,462,002, 4,322,687, 4,306,196, Elektor, 11/98, Applikator, MLX90308, Programmierbares Sensor-Interface, [programmable sensor interface], pp. 72-75, 1998, P. D. Wilson et al., Universal sensor interface chip (USIC): specification and applications outline, Sensor Review, Vol. 16, No. 1, pp. 18-21, ISSN 0260-2288, 1996 disclose electric circuits for voltage offset compensation of a voltage signal on an input of an operational amplifier.